FIG. 2 of the accompanying drawings shows the concept of a conventional tracing control system. A stylus 142 is held in contact with a model 143 to be copied. Amounts .epsilon.x, .epsilon.y, .epsilon.z of displacement of the stylus 142 along respective axes are detected by a tracer head 141. A combined amount ##EQU1## of displacement of the stylus 142 is calculated by a combining circuit 131. An adder 132 then calculates the difference .DELTA..epsilon. between the combined amount .epsilon. of displacement a reference amount .epsilon.0 of displacement and a normal-direction speed signal generator 133 produces a normal-direction speed signal Vn by multiplying the difference .DELTA..epsilon. by a predetermined gain. A tangential-direction speed signal generator 134 generates a tangential-direction speed Vt from the difference .DELTA..epsilon. and a commanded tracing speed.
A switching circuit 135 selects amounts of displacement along the respective axes in a commanded tracing plane. A tracing-direction calculating circuit 136 employs the selected amounts to calculate a cosine cos .theta. and a sine sin .theta. of a profiling direction .theta.. An axis speed signal generator 137 employs Vn, Vt, cos .theta., and sin .theta. to generate axis speed signals Vx, Vy, Vz for the respective tracing axes. Based on the axis speed signals, the stylus 142 is moved on the surface of the model 143. A cutter head 162 is also moved at the same speed as the stylus 142 to machining a workpiece 163.
In the conventional tracing control system, however, there is a certain limitation on tracing speeds. If the tracing speed is excessively high, the cutter head 162 tends to bite into the workpiece 163 in regions where the shape to be copied abruptly changes.
There has been developed a digitizer as a means for solving the above problem. The digitizer reads digital positional data of the positions of the controlled axes from time to time. The digitizer linearly approximates the model surface based on the positional data, and also processes data to prevent the cutter head from biting into the workpiece, such as by temporarily interrupting the reading of positional data in the regions where the shape to be copied abruptly changes, thereby generating NC data. The NC data are then supplied to a computerized numerical control system for machining the workpiece.
The process which employs the digitizer, however, requires an increased number of steps and is not efficient since the model is traced and then the workpiece is machined.